The overall goal of this proposal is to investigate the role of endocannabinoid (ECB) signaling in regulating distinct cortical inhibitory synapses. Physiological brain development and function are dependent on a fine-tuned balance of synaptic excitation and inhibition. As such, dysregulation of inhibition has been implicated in a variety of neurodevelopmental disorders. The cellular targets and consequences of GABAergic signaling in adult animals is well understood, however the specific interneurons modulated by ECB signaling remains poorly studied. Cortical inhibition is mediated by a diverse group of GABAergic interneurons (INs), including cells co-expressing the calcium binding protein parvalbumin (PV), the peptide transmitter somatostatin (SOM), or the serotonin 5HT3a receptor. Cortical INs differ in their synaptic targets, with PV-INs making synapses onto the perisomatic and proximal dendritic regions of target pyramidal neurons and SOM-INs making connections onto the dendritic arbors. A subset of 5HT3a-INs which express the vasoactive intestinal peptide (VIP), innervate dendrites of pyramidal neurons and other interneurons. This subcellular localization allows PV-INs to regulate the magnitude and timing of PN spike output and SOM-INs to regulate dendritic spine and shaft calcium influx along with glutamatergic synaptic plasticity. While the birth and migration of GABAergic INs is well understood, there remain gaps in our knowledge of the molecular and cellular mechanisms that influence inhibitory signaling. ECBs influence synaptogenesis and long-term plasticity of both excitatory and inhibitory connections. Although data has shown coexpression of the cannabinoid type-1 (CB1) receptor with SOM or VIP, colocalization of the CB1 receptor within the various interneurons has not been studied and will be addressed in this proposal. ECB release in the cortex modulates presynaptic GABA release and can drive both short- and long-term depression of inhibitory transmission, however the identity of the presynaptic INs sensitive to ECB activity is not well-characterized. Moreover, the consequences of CB1 receptor loss from distinct interneurons is unknown. In this proposal, we seek to determine how ECB signaling modulates GABAergic signaling mediated by distinct IN populations. We hypothesize that ECB signaling modulates cortical activity, with a key role in influencing plasticity mediated by dendritic targeting interneurons. We propose a novel combination of tools including electrophysiology, 2-photon laser-scanning microscopy (2PLSM), and optogenetic stimulation of genetically-targeted INs in the mouse primary visual cortex. Our experiments will provide an unprecedented level of insight into the development of GABAergic circuits in the neocortex.